1. Field of the Invention
This invention relates to the field of logging-while-drilling (LWD) well boreholes, and more particularly relates to an apparatus and methods for minimizing measurement errors in LWD formation density measurements.
2. Description of the Related Art
The density of formations penetrated by a well borehole is used in many aspects of the petroleum industry. More specifically, formation density is combined with measurements of other formation properties to determine gas saturation, lithology, porosity, the density of hydrocarbons within the formation pore space, properties of shaly sands, and other parameters of interest.
Methods and apparatus for determining formation density, comprising an isotopic gamma ray source and two gamma ray detectors are known in the art and are often referred to as dual spaced density logs or gamma-gamma density logs. For examples of wireline tools incorporating the technique see U.S. Pat. Nos. 3,202,822, 3,321,625, 3,846,631 3,858,037, 3,864,569 and 4,628,202. The wireline apparatus is normally configured as a logging tool (sonde) for conveying, preferably with a multiconductor cable, along a borehole thereby “logging” formation density as a function of depth. The source and two detectors are typically mounted in an articulating pad device with a backup arm. The backup arm applies force to the articulating pad to maximize pad contact with the wall of the borehole. The sonde responds primarily to radiation which is emitted by the source and scattered by the formation into the detectors. The scatter reaction is primarily Compton scattering, and the number of Compton scattering collisions within the formation can be related to electron density of materials within the formation. Through sonde calibration means, a measure of electron density of the formation can be related to true bulk density of the formation.
Since the dual spaced density measurement technique is based upon a nuclear process, statistical error is associated with the measurement. There is also non-statistical error in the measurement. Although the articulating pad and backup arm tend to position the pad against the borehole wall, the largest source of non-statistical error is generally still associated with the position of the tool within the well borehole, and is generally referred to as standoff error. As used herein, standoff refers to the distance from the outer surface of the sensing section of the tool to the wall of the borehole. The responses of the two detectors are combined in prior art dual spaced density systems using well known algorithms to minimize standoff error.
The dual spaced density systems are available as an LWD system. As in the wireline version of the system, the dominant non-statistical error that arises in LWD formation density measurements results from tool standoff. In prior art LWD systems, see FIG. 1, the source 201 and two detectors 202,203 are mounted in-line on an axial blade 208 having a substantially bit gauge diameter such that the source 201 and detectors 202,203 and their associated windows 204-206 are in close proximity to the wall 207 of the borehole. For example see U.S. Pat. No. 5,091,644. As the blade wears during drilling, the collimating windows typically associated with such tools also wear thereby changing the response of the tool. There is no known technique that measures and corrects for this tool wear in real time. These errors must be calibrated out in a lab environment. Today's drilling technology uses high rotational velocities, drills in-gauge hole, and permits very long, continuous drilling periods. Tool wear can no longer be practically calibrated out in the lab, because measurement errors due to wear become excessive during long drilling runs.
The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by positioning the source and detector in a tool section substantially protected from such wear.